Individual differences both in rate and pattern of aging are large, pervasive and ubiquitous. Individuals are distributed between the extremes of those who remain intact throughout life and those who experience early decline in vitality and functional capacity. Understanding the origins of this multifaceted variability is of fundamental importance both for basic- science understanding of aging and age-related processes and for the development of rational health- and disease-related applications. Quantitative genetics offers an explicit analytical model for the identification of relative contributions of two broad domains of influence on living systems --genetics and environment. The study of monozygotic and dizygotic twins is one of the strongest available methods for decomposing the manifest variance in complex traits into genetic and environmental components. This proposal seeks support for a longitudinal extension of our current grant (AGO8861), "Origins of Variance in the Old-Old: Octogenarian Twins" (OCTO-Twin). The aim of OCTO-Twin is to explore the genetic and environmental bases of variability in age-related variables in the oldest- old. The proposed inclusion of two additional measurement occasions at two-year intervals will provide unique information on the relative contributions of genetic and environmental influences to change and continuity in advanced ages. The longitudinal data will also identify significant numbers of individuals giving evidence of cognitive impairment, which will shed light on the critical question of whether late-onset diagnosable dementias are distinct, categorical conditions or whether they represent the low end of a continuous distribution of cognitive function.